Silabs: allow EM2 sleep for battery end devices

The debug UART held an EM1 requirement to keep its receiver on, which kept sleepy end devices out of EM2 (~1mA vs ~1.5uA). Routers are unaffected: the radio keeps them at EM1 anyway.

Author: eakorolev

docs/changelog_fw.md

@@ -40,6 +40,7 @@ Please describe what you are working on, under ## Upcoming
   - PowerCfg cluster for battery level monitoring
   - PollCtrl cluster to allow settings poll rate via ZCL
   - Add `BT<pin>` config option to enable battery mode
+  - Silabs end devices now reach EM2 while idle (the debug UART no longer holds an EM1 requirement to keep its receiver on)
 
 ### Bugs
 

src/silabs/config/sl_iostream_usart_inst_config.h

@@ -72,11 +72,14 @@
 // <i> Default: 0
 #define SL_IOSTREAM_USART_INST_CONVERT_BY_DEFAULT_LF_TO_CRLF    0
 
-// <q SL_IOSTREAM_USART_INST_RESTRICT_ENERGY_MODE_TO_ALLOW_RECEPTION> Restrict
-// the energy mode to allow the reception. <i> Default: 1 <i> Limits the lowest
-// energy mode the system can sleep to in order to keep the reception on. May
-// cause higher power consumption.
-#define SL_IOSTREAM_USART_INST_RESTRICT_ENERGY_MODE_TO_ALLOW_RECEPTION    1
+// <q SL_IOSTREAM_USART_INST_RESTRICT_ENERGY_MODE_TO_ALLOW_RECEPTION> Hold an
+// EM1 requirement so the debug serial UART can receive console input.
+// <i> Default: 1
+// <i> Keeping the UART receiver on blocks EM2, so a sleepy battery device
+// <i> never really sleeps (~1mA vs ~1.5uA). Set to 0 here: battery end devices
+// <i> reach EM2 and lose only debug-serial input while idle. The Zigbee radio
+// <i> is unaffected -- it still wakes to poll and receive.
+#define SL_IOSTREAM_USART_INST_RESTRICT_ENERGY_MODE_TO_ALLOW_RECEPTION    0
 
 // </h>